Some of the critters you'll see more frequently are Onix, Eevee, Roselia and Burmy -- you may even meet a Shiny Burmy if you're lucky. You'll also encounter more Lickitung, Scyther, Togetic and Ralts in raids, where you can get evolution items, as well. Each of those Pokémon is unique in different ways: Eeevee, for instance, can evolve into multiple species through several methods.
You'll also find Tyrogue, Feebas, Burmy and Happiny hatching more frequently out of two-kilometer eggs. If you're looking to get a certain Pokémon, though, you may want to check out the event-specific Field Research tasks. Those quests will dole out evolution items as rewards, and you can use them to evolve particular monsters.
In addition, lure modules will last one hour for the duration of the event, and you'll get twice the XP for evolving monsters. On December 7th, from 11AM to 2PM in your local timezone, Pokémon Go will also hold an Evolution Event raid day, wherein you can encounter Bulbasaur, Charmander and Squirtle and their evolved forms.